Microwave signals are extremely high frequency (HF) signals, usually in the gigahertz range. They are used to transmit large amounts of video, audio, RF, telephone, and computer data over long distances. They are used in commercial and military applications, including communications to satellites, airplanes and the like. Frequencies are divided into various bands such as the S-band (2-3.5 GHz), Ku-band (10.7-18 GHz), Ka-band (18-31 GHz), and others such as the X-band etc.
Polarization is a characteristic of the electromagnetic wave. Four types of polarization are used in satellite and other transmissions: horizontal; vertical; right-hand circular (RHCP); and left-hand circular (LHCP). Horizontal and vertical polarizations are types of linear polarizations. Linear and circular polarizations are well known in the art. An example of linear polarization is shown in FIG. 1A. A wave is made up of an electric field ‘E’ and a magnetic field ‘M’. When a wave of wavelength ‘k’ is transmitted into free space from an antenna, the orientation of its electric field E with respect to the plane of the earth's surface determines the polarization of the wave. If the wave is oriented such that the E field is perpendicular to the earth, the wave is referred to as vertically polarized. If the ‘E’ field is parallel to the earth's surface, the wave is horizontally polarized, which is the orientation of the ‘E’ field shown in FIG. 1A. Also shown is the magnetic field ‘M’. In both of these cases, the wave polarization remains in the same orientation at all times and is, therefore, referred to as linear polarization. The wave travels in direction ‘C’ along the X-axis.
FIG. 1B depicts the alternative to linear polarization, referred to as circular polarization. In this kind of electro-magnetic emission, the ‘E’ field is no longer confined to a single plane, but consists of equal-amplitude horizontally and vertically polarized components, which are phase-shifted by 90°. It can be readily seen that the vectors of both the ‘E’ and ‘M’ fields are rotating in a clockwise direction (if viewed from behind an antenna). This rotation is called RHCP. For every cycle of the transmitted wave, the ‘E’ and ‘M’ fields will rotate a full 360°. An observer (standing behind the antenna) would “see” the rotation vector in this drawing rotating in a circular clockwise motion R and moving in direction C along the X-axis. The type of polarization is controlled by the design of the antenna feed assembly.
Multi-frequency band feeds exist that have the ability to send/receive more than one frequency and are usually designed for frequency bandwidths within one or more of the aforementioned bands.
A typical multi-frequency band feed without tracking (prior art) is shown in the block diagram of FIG. 2A and consists of a waveguide assembly 20A with the following components:                1. Multi-frequency band horn 22 to produce the desired radiation pattern characteristics, where an input signal is received or an output signal is transmitted.        2. Behind the horn, first common junction 24 with appropriate filters is used to separate out the two orthogonal linear polarizations of the lowest frequency band without impacting any of the higher frequency bands. Filters include first low pass filter 26 (LF filter) to filter the lowest frequency range and first high pass filter 36 (HF filter) to filter the higher frequency ranges. If circular polarization is required, first 90° polarizer 28 (low frequency (LF) polarizer) attaches to both first (LF) waveguide port right hand circular polarization (RHCP) 32 and LF waveguide port left hand circular polarization (LHCP) 34. Ports 32, 34 can also be used for horizontal or vertical polarization respectively if circular polarization is not required. In this case the 90° polarizer 28 is not required.        3. Second common junction 38 with appropriate filters is used to separate out the two orthogonal linear polarizations of the next lowest frequency band without impacting any of the higher frequency bands. Filters include second low pass filter 42 to filter the second lowest frequency range and second high pass filter 52 to filter the next higher frequency range. If circular polarization is required, second 90° polarizer 44 attaches to both second waveguide port RHCP 46 and second waveguide port LHCP 48. Additional common junctions, not shown, are added for additional frequency band requirements. Ports 46,48 will also be used for horizontal or vertical polarization respectively if circular polarization is not required. In this case second 90° polarizer 44 is not required.        4. An Ortho-Mode Transducer (OMT) is used after the last common junction to separate the two orthogonal linear polarizations of the highest frequency bands. If circular polarization is required, a polarizer can be used immediately in front of the OMT. A combined OMT/Polarizer 54 (e.g. a Septum Polarizer) is shown instead of a separate OMT and polarizer. OMT/Polarizer 54 comprises high frequency RHCP port 56 and high frequency LHCP port 58.        5. A four port feed would have one common junction whereas a six port feed would have two common junctions and so forth.        6. If a dual band feed were used, then OMT/Polarizer 54 would be placed after the first common junction and first high pass filter.        7. If additional frequency bands are present, OMTs, OMT/Polarizers, or more junctions are used in the proper sequence as described above to separate higher frequency bands.        8. If tracking is required, aforementioned waveguide assembly 20A is modified to waveguide assembly 20B as shown in FIG. 2B. This modification adds a higher order mode coupler (e.g. TE21 or TM01) 25 placed between the Multi-frequency band horn 22 and the first common junction 24, to extract a difference signal 23 used for tracking purposes. All other functions depicted in FIG. 2B are as described above for FIG. 2A.        
Further references to a multi-frequency feed as noted herein imply a feed with single/dual linear/circular polarizations with/without tracking. The term “microwave” refers to signals with a frequency ranging from 1 giga hertz to 1,000 giga hertz.
The traditional way of producing a multi-frequency band feed system is to produce each component separately, and join them together by use of flanges, brazing or other techniques. An assembly of separate components can be expensive to produce, requires more space, and demands many flange connections, which can degrade the performance of the system.
Prior art of feed system designs are illustrated and described in U.S. Pat. No. 4,228,410 issued Oct. 14, 1980 to Kenneth R. Goudey, assigned to Ford Aerospace and Communications Corporation. Another design is illustrated and described in U.S. Pat. No. 6,700,548 B1 issued Mar. 2, 2004 to Ming Hui Chen, assigned to Victory Industrial Corporation.
The problem with the prior art feed U.S. Pat. No. 4,228,410 is that it requires many components, which result in a very long feed (several feet long for C-band) and is not cost effective to manufacture because of the complexity of the individual components. The large number of flange connections can also cause negative effects on electrical performances.
The problem with prior art feed U.S. Pat. No. 6,700,548 B1 is that the layout still results in a long feed. The assembly is made by joining four separate sections, which are not necessarily joined along the zero current line. Failure to join components along the zero current line can result in degraded electrical performance.
Large physical size of a feed assembly is a problem for many applications including satellites, airplanes, military craft, etc. The present invention solves the problems of size, for example the present invention would reduce the size of a C-band waveguide from over several feet long to less than one foot long. The present invention provides for ease of manufacture and optimizes the efficiency with respect to signal losses.